1. Field of the Invention
The present invention relates to an optical fiber unit installed in a previously laid cable conduit by use of air pressure.
2. Description of the Related Art
There has been known a so-called air blown fiber installation system. In such a system, a “pipe cable” (hereinafter, referred to as a conduit) obtained by aggregating at least one polyethylene pipe (inner diameter: about 6 mm) as a cable conduit is previously laid. An optical fiber unit was blown through the conduit by a drag force and a driving force caused by a difference of pressure of compressed air to install optical fibers, and thus a transmission channel of optical communication is constituted.
In the optical fiber unit for use in such an installation system, a coating thereof is formed of foamed resin for the purpose of reducing frictional resistance with an inner surface of the conduit or the other optical fiber unit in the conduit, buffering an external force to reduce a transmission loss of the optical fiber due to the external force, and reducing the weight of the optical fiber unit for blowing the same for a long distance.
In order to pick up an optical fiber wire from the optical fiber unit, it is necessary to strip part of the outer coating, particularly by the use of a tool.
The most general stripping method is to cut off the tip portion of the coating by the use of a cutter. However, in use of the cutter, there is a possibility of accidentally damaging a optical fiber wire.
Therefore, the Japanese Patent Laid-Open No. Hei 8-271772 publication disclosed an optical fiber unit including a first coating layer (nylon, silicone, etc.) and a second coating layer (foamed polyethylene) which are stacked on the outside of a unit core formed of jacketed optical fibers. The optical fiber unit is provided with a rip cord within the first coating layer.
Recently, an optical fiber unit applied for the air blown fiber installation system has been developed, in which an optical fiber wire is coated with ultraviolet curing resin (hereinafter, referred to as UV resin) to form an inner layer, and foamed polyethylene or the like is directly stacked on the outside of the inner layer.
However, with this optical fiber unit, a problem arises that side pressure is applied to the optical fiber wire due to shrinkage stress of the foamed polyethylene coating layer, which is involved in cooling after extrusion of the foamed polyethylene layer serving as an outer coating, and due to an external force caused by roughness of the inner surface of the foamed polyethylene coating layer, thus causing a transmission loss.
Also, as the result of giving high rigidity to the UV resin serving as an inner layer, the transmission loss is reduced. However, a problem remains that it becomes difficult to peel off the coating of the optical fiber unit to pick up the optical fiber wire for connection thereof, that is, so-called wire stripping work.
Also, when an optical fiber is laid by the air blown fiber method, for example, an optical fiber unit for air blown fiber installation as shown in FIG. 1 has been hitherto used as the optical fiber.
This optical fiber unit for air blown fiber installation includes an inner coating layer 5 made of the UV resin on the outer periphery of an optical fiber ribbon aggregate 4 having a plurality of optical fiber ribbons 3 stacked on each other. The optical fiber unit further includes an outer coating layer 6 made of foamed polyethylene or the like on the outer periphery of the inner coating layer 5. Each optical fiber ribbon 3 includes four optical fiber wires 1 aligned parallel to each other in a row and unitized in a tape form with a block coating layer 2. For the optical fiber wire 1, a single mode optical fiber wire or a graded index optical fiber wire is used. The number of optical fiber wires 1 constituting the optical fiber ribbon 3 and the number of optical fiber ribbons 3 stacked can be properly determined.
Meanwhile, as the demand for the optical fibers has increased, the optical fibers have been laid in various locations. Accordingly, the optical fiber unit for air blown fiber installation requires further improvement in air blowing properties thereof such as an increase in length in which the optical fiber unit can be blown, shortening of a period of time in which the optical fiber unit can be blown, and air blowing through a channel such as a pipe with more curves.
In the conventional optical fiber unit for air blown fiber installation as shown in FIG. 1, since the inner coating layer 5 is formed of the UV resin to be solid, the inner coating layer 5 must be heavy and the weight per unit length of the optical fiber unit becomes large. Therefore, this optical fiber unit is not excellent in the air blowing properties very much.